The present invention relates to an output circuit, and more particularly, to a high-speed output circuit for matching impedance.
Referring to FIG. 7, a conventional output circuit of this type includes a PMOS transistor 106 and an NMOS transistor 206. The output circuit is connected to a transmission line. In general, it is important to match impedance between the transmission line and a load (the output circuit). MOS transistors 106 and 206 have resistor components, however, since it is difficult to match the impedance by using only the resistor component of the MOS transistor itself, a resistor is built in the LSI or added to the exterior of the LSI to match the impedance. Thus, the output circuit is provided with a termination resistor 308 for matching the impedance between the transmission line and the output circuit. The output circuit inputs an input signal from an input terminal 508 and outputs an output signal to a output terminal 506 via resistor 308.
However, in this embodiment, since the resistance value of the MOS transistors is influenced by variations in temperature or power voltage or a manufacturing process, rather than that by the resistor 308, it is necessary to increase the resistance of resistor 308. At the same time, the resistance component of the MOS transistors should be decreased. As a result, the gate widths of the MOS transistors 106 and 206 are increased. Thus, a problem arises because the size of the MOS transistor increases. In general, since a LSI has a plurality of output circuits, the size of the LSI also increases. Furthermore, as the size of the MOS transistor increases, another problem is produced because noise and power consumption increases due to an increase of the through-current of MOS transistors 106 and 206.